Why Don’t Birds Fall Over When They Take Off?

Credit: NickVorobey.com/Adobe

By Ben Parslew

An analysis of the biomechanics of the powerful jump of a bird taking flight gives inspiration for the future of agile robots.

A plump pigeon with fluffy grey feathers is perched comfortably on the window ledge of my apartment. Its head twitches jerkily from side to side as its fiery orange eyes scan the pavement below for a dropped crumb.

But suddenly a car door slams shut, and the echoing thud panics the bird into action, lunging into the air with frantic clapping wings, a rapid ascent and then a swoop over the traffic below before a nimble and precise landing on the building opposite. This whole maneuver lasts less than two seconds but is so rich in physics that understanding it has kept scientists busy for decades.

Our latest research on bird flight, which has been published in Royal Society Open Science (https://goo.gl/nEvWhc), focused on just the very first instance – the jump – and we are now able to decipher how birds propel themselves from ground to air. Using a combination of mathematics, computer simulations and zoological data, we can explain the mechanics of jumping and how birds take off with such precision.

This brings us a step closer to understanding how the ancestors of living birds first jumped into flight more than 100 million years ago. It also gives us inspiration for designing the next generation of highly agile robots that will roam our planet in the future.

The challenge with studying jumping is that it is quick. The powerful leg-driven jump of a pigeon lasts only around one-fifth of a second. For smaller birds like sparrows and finches it is even more brief.

During this time a lot happens. A bird prepares for its jump by crouching, lowering its body and centre of gravity in what is known as a countermovement. It then grips its feet into the ground and leans forward over its toes. The legs start to extend and accelerate the body forwards and away from the ground, while simultaneously pitching the body and head upwards and raising the wings. As the legs extend further and further towards their limits, the toes start to disengage as they give one final push from the ground and the bird enters the air. The first powerful wingbeat flaps down, thrusting back a cloud of air and propelling the bird forwards.

This rapid flow of events makes studying bird jumps particularly challenging, and forces us to use technology to slow things down. High-speed cameras have been essential in enabling us to capture and review slow-motion videos of birds jumping and flying. There is now a wealth of footage available, some of which dates way back to the end of the 19th century.

But the problem with video and photographs is that they only capture what’s on the surface of an animal and miss out on all of the juicy details beneath. Birds are covered in soft feathers that cloak the body and the limbs, obscuring the motion of the joints, bones, muscles and tendons that we need to see in order to understand the physics.

This is particularly apparent with the leg, where very little of it is actually exposed beyond the feathers. That funny looking backwards-knee that you see in a bird’s leg? That’s actually an ankle. The knee is higher up, close to the body, surrounded by feathers and generally invisible to the camera. Trying to capture the thigh and hip that are tucked away even further? Forget about it.

Thankfully, our ability to visualise the movement of animal limbs has increased dramatically through a new technique developed at Brown University called X-ray Reconstruction of Moving Morphology (XROMM). This X-ray movie effectively cuts away the animal’s outer surface, including the feathers, revealing the internal structure and allowing us to see the motion of the bones and soft tissues. By combining two X-ray videos, the XROMM method provides us with a 3D virtual moving skeleton.

XROMM has already been used to visualise a range of animal skeletal motions, from the leg bones of walking crocodiles to the jaws of sucking fishes. It has even been used to examine how baby birds can learn to climb steep slopes by flapping their wings in time with their footsteps.

One of the most recent studies using XROMM examined how diamond doves and zebra finches take off from perches. Dr Pauline Provini at the Muséum National d'Histoire Naturelle in Paris used this technique to visualise the fine movements of the head, trunk and leg bones of the birds as they jump.

These beautifully clear reconstructions were precisely what we had been looking for in our own research on jumping. Before then, our analyses had relied on much less accurate recordings of bird legs taken from videos.

Furthermore, all of the previous measurements had examined birds jumping from the ground, but the mathematical tools we had been developing had shown us that gripping of the foot when taking off from a perch has a significant impact on the jump. With this new XROMM-enabled data at our disposal, we could now include the effects of foot gripping into our research and gain a more complete understanding of jumping physics.

Our approach to examining jumping was founded on the concept of stability. It is well accepted that the ability to remain stable and not tip over is a fundamental, if perhaps obvious, criterion for successful locomotion. This is even more apparent in manmade devices, where the clatter of a toppling robot hitting the deck is a clear indication that something has gone wrong.

So it’s no surprise that there has been considerable effort made among the designers of walking robots to understand stability, and the mathematics of walking stability has now been around for several decades. With just one simple equation, the maths shows us exactly how a robot or animal needs to move its body and limbs as it walks in order to remain upright.

So why can’t the same rules be applied to jumping? It turns out that they can, and this is something that our recent study on jumping take-off found out. Using Provini’s brand new XROMM virtual skeleton, and applying traditional mathematics of stability from the 1970s, we could see exactly how birds remain stable and how they avoid tipping over when they jump.

We did this by creating a computer simulation of a virtual jumping bird skeleton with an added stability indicator. Just like a fuel gauge on a car, this warned us whenever the bird was close to falling over. As we replayed the jump we saw the indicator light up as the bird crouched down in its countermovement, alerting us to a potential topple and faceplant. But just in time the bird pitched its body upwards, as if trying to perform a mini-backflip. This movement gives the precise impulse needed to avoid toppling forward and to guide the bird into a smooth and elegant launch.

We also found that birds can use the grip of their feet not only to prevent slipping but also to achieve more extreme take-off. This is most apparent when taking off from perches, where the toes can wrap snugly around the perch to create lots of gripping force. The grip provides leverage to rapidly pitch the body during the jump, keeping the bird balanced and allowing take-offs at extremely shallow or extremely steep trajectories. This provides a huge advantage to birds in the wild to escape from incoming predators, such as raptors dive-bombing from above.

But what about the wings? When a bird takes off, the main thing that we see and hear is the flapping of the wings. In terms of the physics, the movement of the wings during take-off is actually a bit of a distraction, at least at the very beginning.

Several previous studies have already demonstrated that the legs are the main thrust provider for take-off, with the wings contributing only around 10% of the force needed to get off the ground. Our research hit another nail in the coffin for wings in take-off, showing that they don’t do much for stability either. Many birds could take off in pretty much the same way if they didn't have wings at all.

So the reason behind the rapid upward movement of the wings during take-off is simply as preparation for when they will be used – the first powerful flap that happens once the bird is airborne. But during the jump, it’s all about the legs.

How is this knowledge useful? This is the kind of question fellow engineers throw my way when I talk about birds and jumping, and this is where I retaliate by bringing up robots. The ability of robots to walk, run and climb seems to have accelerated in the past 10 years. Every jaw-dropping video posted by Boston Dynamics makes me feel like we are finally in the future, where robots move more like humans. But now the real future is to build robots that exceed human physical abilities. Ones that can jump higher, swim faster, grip harder. An exciting route to doing this is by mimicking the athleticism seen in other parts of the animal kingdom.

The current research movement in agile robots has highlighted a need for robots that are faster and more dextrous than humans, traversing cluttered nuclear radiation sites, negotiating a rocky Martian landscape or even climbing debris in a natural disaster. All of these require machines that are far more nimble than the fittest human athlete or gymnast.

Imagine a robotic frog that could leap over a giant boulder ten times its own size. Or a robotic cat that could pounce and claw its way up a wall. Or a robotic bird that can sit perched and watching, ready to jump at the first sign of danger, and to swoop and turn and glide and land with pinpoint precision.

These are the agile machines of the future, and they will be made possible by understanding animal mechanics.

Ben Parslew is a Senior Lecturer in Aerospace Engineering at The University of Manchester.